Refactor the writable warm index feature flag

[neetikasinghal]

Refactor the writable remote index feature flag to include the tiering functionality as well. Considered the following options:

1. TIERED_REMOTE_INDEX
2. TIERED_WRITEABLE_INDEX
3. TIERED_WRITEABLE_REMOTE_INDEX (Preferred)
4. TIERED_WRITEABLE_WARM_INDEX
5. TIERING_EXPERIMENTAL